2011 XK Convertible

I had my main battery replaced at a Jaguar dealership recently. I have been going to them for several years.

After having the battery replaced, I monitored the battery voltage on a few 20-to-30 mile trips at highway speeds. I did this over a period of several days. Immediately after starting the engine the voltage would rise to 13.6 volts but only for a minute or so. Then the voltage would drop to 12.7 volts. At highway speeds the voltage would fluctuate between 12.7 and 12.9 volts. Immediately after engine shutdown the voltage usually stabilized around 12.6 volts. The car was always locked whenever it was not running. After sitting overnight, the battery voltage was usually 12.4 volts.

Knowing that sometimes new batteries aren’t always fully charged when installed, I connected a CTEK 4.3 charger to my battery for a few days. The positive lead was attached to the positive battery terminal and the negative lead was attached to a chassis ground. I had four lights illuminated on the CTEK charger within minutes and seven lights illuminated by morning. That indicated to me that my battery was fully charged. My normal battery voltage is 12.4 volts when the car has been sitting overnight. That seems a little low to me. After engine start, the voltage would rise to 13.6 volts. That seemed reasonable, but then the voltage would drop to 12.7 volts after only a couple minutes. I thought the voltage would stabilize between 13 and 14 volts while driving but that was not the case. 13.6 volts after engine start for only one or two minutes didn’t seem right to me. I also thought that 12.7 to 12.9 volts while driving at highway speeds was low.

I never monitored my voltages before so I have nothing to compare. I don't drive the car regularly and I replace the battery about every three years. I would occasional connect my CTEK charger. I'm coming to the conclusion that I need to keep the CTEK charger connected when I am not driving my car.

The Jaguar dealer told me that they tested my alternator before replacing my battery and it passed the alternator test. The service manager told me that the voltages I am seeing are normal for the XK. He also mentioned that other models of Jaguars may be different.

Should I be concerned about my alternator?

Is there a way of checking the BMS module on the negative terminal of the battery?


I would be interested in knowing what voltages others are seeing on their XK’s

1. Voltage while charger is connected and in maintenance mode?

2. Voltage before start?

3. Voltage immediately after a start?

4. Voltage at highway speeds?


I welcome your comments.

 

I have purposely not answered your requests because;
Firstly, what you are reading when the engine is running, is the charging voltage. It is not the battery voltage.
Secondly, the alternator provides a variable output dependent on a number of factors. This is laid out in the workshop manual.
Lastly, a fully charged 6 cell agm battery in excellent condition can read 13.0 v, after sitting all night without external charge. Over its lifespan, this value will dwindle down. Different people with different standards will replace them at some value below 12.4 v, as there is sufficient resistance and insufficient voltage to run the electronics of a modern vehicle.
Cheers.

 

I appreciate your reply. I'm just not used to seeing voltages this low in cars I have owned.

1. Point well taken concerning the distinction between charging voltage vs battery voltage. Poor choice of words on my part. In any case what I am referring to is the voltage measured between the positive terminal of the battery and the chassis ground regardless of the source.

2. I understand that the alternator will provide variable output depending on conditions. My question is more of a “big picture” one. I would think that in all cases 12.7 to 12.9 volts would be considered low when driving at highway speeds. Can you conceive of a situation where that would be an acceptable voltage?

3. Being as this is a new and fully charged flooded battery, I would think that 12.4 volts would be considered “low” after sitting all night without an external charge. I was expecting to see more like 12.6 to 12.7 volts. Just an observation on my part from other cars I have owned.

4. Could a faulty BMS module account for voltages being lower than they should be? It’s just hard for me to believe 12.7 to 12.9 volts would charge the battery at an acceptable rate.

5. When testing the alternator, should the BMS be out of the circuit?

6. Is there a way to test the BMS module?

 

2. Speed has little to do with it.
3. I concur.
4. Sure... you need a jaguar code reader to confirm jaguar specific codes. But I believe the check engine light would illuminate as it is ecm monitored.
5. Follow the pinpoint tests. These are in the workshop manual. I suspect you are chasing a red herring unless you do in fact have a check engine light. For most of us, we replaced the alternator when it became noisy. Though it was still operational.
6. Codes.

 

For what it is worth the numbers you quote appear low to me but there are a number of variables.
My 2012 XF has a Jaguar battery, made in Germany by Varta. It is 6years old and I bought it for my 2004 Vanden Plas and swapped it to my 2012 XF in 2020
I monitor the voltage on the battery continuously when the car runs and when the car is in the garage I have a permanently installed C-TEC battery maintainer hooked up.
I typically see 14.3-14.6V on start up and 13.7-14.1V after a few minutes of running. On engine off I typically see 12.7-12.8V these days.





The pictures show the battery (before the C-TEC was installed) and my monitor with engine oil temp, engine coolant temp and battery voltage, all from the diagnostic connector after a trip and before shutting the engine off.

 

I've been dealing with similar issues. I replaced my alternator and BMS negative cable to no avail. I've started two threads on this subject. I've had my car inspected and tested at the dealer. It runs perfectly with no codes. I too found the low voltage readings disconcerting. However, after much research, I believe these cars are just built this way. I always connect the battery maintainer to the car after use because I don't like the low voltage readings.

 

I’m having very similar problems with my 2012 XK. The battery will flatten itself over a week or two. Both battery and alternator were replaced 6 months ago as a way to treat this but the issue still persists.

the alternator is capable of supplying full voltage to the battery but for some reason it’s only giving the battery 12.8-13v. It’s like the BMS is looking at the battery and saying “yup, that’s charged” and allowing it to discharge.

 

My 2014 is behaving exactly as you described. I replaced my alternator and BMS negative cable to no avail. I have a new AGM battery installed.

 

have you had the BMS reset by a dealer/someone with an icarsoft?

 

I have had all kind of wacky “ghost of Lucas electronics” haunting my 2010 XKR for years. I’ve replaced many a dead battery. Nowadays I plug the car into a battery maintainer when not in use and the majority of my ghosts disappear and my batteries last much longer.

 

Yes, I had the dealer reset the BMS, replaced the alternator, and then replaced the BMS to no avail. Other than the battery being depleted the car works flawlessly. I use a battery maintainer when not in use.

 

Never underestimate the value of cleaning battery terminals. And don't trust that the "professionals" remembered to do it.
The wire brush tool for the specific purpose is available for less than $5 at the china tool store.

Parasitic drains, where modules failed to go to sleep, were the subject of early TSBs and software updates.

However, my 2010 XK 135K was exhibiting the "lazy alternator" behavior described by My First Jaguar. A late model, low milage replacement alternator restored proper operation: 14V+ on start-up, dropping to mid to 13s in normal operation.

Replacement of the alternator made the difference. But, was it the replacement alternator or the replacement procedure?